1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. Particularly, the present invention is directed to a method of forming an insulating film for isolating a wiring layer.
2. Description of the Related Art
An insulating film for electrically isolating an element wiring is used in a semiconductor device. It was customary in the past to use as an insulating film a SiO.sub.2 film formed by thermal oxidation of a Si substrate or a SiO.sub.2 film formed by chemical vapor deposition under a reduced pressure or atmospheric pressure using gaseous materials such as silane and tetraethoxy silane (TEOS). For insulation of, particularly, Al wirings, used is a SiO.sub.2 film formed by plasma CVD using TEOS and O.sub.2 because the SiO.sub.2 film can be formed under such a low temperature as about 400.degree. C.
In recent years, delay of signal transmission, which accompanies the miniaturization of the element, has come to be worried about. To be more specific, the interval between two adjacent wirings is shortened in accordance with miniaturization of the element, leading to an increased capacitance between the wirings and, thus, to the delay in the signal transmission. The delayed signal transmission obstructs a high operating speed of the semiconductor device so as to give rise to one of the causes of inhibiting the performance improvement of the semiconductor device. It follows that it is important to diminish the dielectric constant of the insulating film interposed between the two adjacent wirings.
The SiO.sub.2 film formed by the conventional plasma CVD method has been found to have a relative dielectric constant of 4.0 to 5.0. This makes it interesting to introduce F into SiO.sub.2 in an attempt to lower the dielectric constant.
For example, it is described in Published Unexamined Japanese Patent Application No. 2-77127 that F is introduced into SiO.sub.2 by means of ion implantation so as to lower the dielectric constant of SiO.sub.2. In this method, however, it is necessary to set the dose of F at 1 .times.10.sup.19 atoms cm.sup.-3 or more, giving rise to the problem that the ion implantation takes a long time. In addition, it is necessary to apply a heat treatment at such a high temperature as at least 600.degree. C. in order to stabilize F within the SiO.sub.2 layer. It follows that the SiO.sub.2 layer formed by the method disclosed in this prior art cannot be used for the electric isolation of Al wirings.
A CVD method under room temperature, which uses FSi(OC.sub.2 H.sub.5).sub.3 and H.sub.2 O, is reported in, for example, "T. Homma et al., IEEE IEDM, pp. 289 (1991)". In this method, however, it is difficult to control the F concentration in SiO.sub.2. In addition, a serious difficulty is brought about that the formed SiO.sub.2 film is highly hygroscopic.
Also known is a method in which an aqueous solution of boric acid is added to a supersaturated aqueous solution of H.sub.2 SiF.sub.6 so as to utilize the reaction given below for forming a SiO.sub.2 film: EQU H.sub.2 SiF.sub.6 +2H.sub.2 O.fwdarw.SiO.sub.2 +6HF
It is reported in Published Unexamined Japanese Patent Application No. 3-97247 that 5 at % of F is contained in the SiO.sub.2 film formed by the method noted above, with the result that the relative dielectric constant of the SiO.sub.2 film is rendered smaller than 3.9, which is the specific dielectric constant of a SiO.sub.2 film formed by thermal oxidation. In this method, however, it is difficult to control the F concentration in the SiO.sub.2 film. In addition, the growing rate of the SiO.sub.2 film is as low as about 1 nm/min.
Further, a method of improving the step coverage of a SiO.sub.2 film by using tetraethoxy silane (TEOS), O.sub.2 and NF.sub.3 as source gases is disclosed in "Proc. 2nd Int. ULSI Science and Tech. Symp. ECS Proc. (1989)", though the dielectric constant and hygroscopic property of the SiO.sub.2 film are not referred to at all in this publication.
As described above, where the interval between two adjacent wirings is diminished in accordance with miniaturization of the element, the capacitance between these wirings is increased so as to bring about the problem that the signal transmission is delayed. To overcome the difficulty, it is proposed to introduce F into the SiO.sub.2 insulating film so as to lower the dielectric constant of the insulating film. In the prior art, however, it is difficult to control accurately the F concentration in the SiO.sub.2 film. It is also difficult to enable the F-containing SiO.sub.2 film to be less hygroscopic.